I have a small Windows 2008 R2 AD network with about 60 users. I'd like to add some storage space to the network, but I'd prefer an open source solution to more Microsoft. Don't get me wrong, AD is great for managing Windows clients, but all I want to add is a file server.

I've considered SAMBA, but I don't really know how well it integrates with Active Directory. For example, would I have to maintain two sets of users (AD and Samba)?

Also, I've read good things about both FreeNAS and OpenFiler, but very little about truly integrating them into AD.

I really only have two requirements:

1. Software must be open source.
2. Security must integrate with my existing Active Directory.

I'm even up to buying a commercial appliance running OS software if it integrates easily.

Basically, I'm looking for direction from those of you who've done something similar.